CB1 receptors (CB1R) mediate the psychoactive effects of delta9-tetrahydrocannabinol (THC), the main psychoactive ingredient in marijuana, and therapeutic effects of cannabinoids (CBs) in the CNS. However, therapeutic use of CBs is limited by undesirable side effects and development of tolerance and dependence with chronic use. These effects are mediated by CB1R in diverse brain circuits, but understanding of the functional n euro an an atomy of CBs has been limited using conventional autoradiographic image analysis. Chronic CB effects occur by G-protein receptor kinase/beta-arrestin mediated uncoupling of CB1R from Gproteins (desensitization) and/or reduction in CB1R density (downregulation). These adaptations vary regionally in the CNS with regard to factors such as magnitude and time course for development and recovery. Few studies have investigated region-specific adaption produced by chronic administration of different CBs. Moreover, the chronic effect of treatment with endocannabinoids or stable analogs is unclear. The widespread CNS distribution of CB1R and their diverse neuromodulatory roles suggest a more intricate profile of regional adaptation than previously appreciated. In addition, regional differences in adaptation likely result from differential co-localization of CB1R with regulatory proteins, such as beta-arrestin-2. These studies will test the hypothesis that chronic administration of CB agonists produces differential regional patterns of CB1R adaptation. Further, because region-specific adaptation is dependent on interaction with different regulatory proteins, based on their anatomical co-distribution with CB1 R, we hypothesize that CB1R adaptation will be decreased in Beta-arrestin-2 null mice. We propose to examine CB1R adaptation using a novel 3D voxel-based imaging analysis in reconstructed mouse brains, which will provide an unprecedented level of anatomical analysis of autoradiographic data. This will be examined on an unbiased whole-brain basis, using agonist-stimulated [35S]GTPgammaS and [3H]ligand autoradiography to assess CB1-mediated G-protein activity and receptors, respectively. Because we will investigate CB1R adaptations in the whole brain and establish significant changes at the voxel level, we predict that our proposed studies will provide a comprehensive functional anatomical study of CB1R function that will offer new insight into the CB system. We predict that these adaptations will also be associated with the development of tolerance to cannabimimetic effects. PUBLIC HEALTH RELEVANCE: CBs offer potential therapeutic benefits to public health in the management of many debilitating medical conditions. For patients who are unresponsive to existing treatments, CB-based therapy offers alternatives in their clinical management.